PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Net Periodic Cost (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2014		Sep. 30, 2013	Sep. 30, 2012
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period		$ 10.2	[1]	$ 9.2	$ 9.2
Interest cost on projected benefit obligation		24.5	[1]	17.0	19.4
Expected return on plan assets		(27.2)	[1]	(19.4)	(19.6)
Amortization of prior other comprehensive income		0.4	[1]	0	0
Amortization of prior service cost		0.5	[1]	0.5	0.6
Amortization of actuarial loss		7.1	[1]	10.7	9.0
Loss on lump-sum settlements		1.5	[1]	27.0	20.1
Sub-total		17.0	[1]	45.0	38.7
Regulatory adjustment		10.4	[1]	(27.5)	(18.6)
Net pension cost		27.4	[1]	17.5	20.1
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period		11.3	[1]	10.2	8.1
Interest cost on projected benefit obligation		8.9	[1]	5.2	5.5
Expected return on plan assets		(7.3)	[1]	(4.5)	(4.0)
Amortization of prior other comprehensive income		(0.2)	[1]	0	0
Amortization of transition obligation		0	[1]	0.1	0.1
Amortization of prior service cost		0	[1]	0	(2.1)
Amortization of actuarial loss		6.0	[1]	5.3	4.3
Sub-total		18.7	[1]	16.3	11.9
Regulatory adjustment		(9.6)	[1]	(6.8)	(2.4)
Net pension cost	9.1	9.1	[1]	9.5	9.5
Laclede Gas | Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period		9.7		9.2	9.2
Interest cost on projected benefit obligation		24.0		17.0	19.4
Expected return on plan assets		(26.5)		(19.4)	(19.6)
Amortization of prior service cost		0.5		0.5	0.6
Amortization of actuarial loss		7.1		10.7	9.0
Loss on lump-sum settlements		1.5		27.0	20.1
Sub-total		16.3		45.0	38.7
Regulatory adjustment		10.4		(27.5)	(18.6)
Net pension cost		26.7		17.5	20.1
Laclede Gas | Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost – benefits earned during the period		11.2		10.2	8.1
Interest cost on projected benefit obligation		8.7		5.2	5.5
Expected return on plan assets		(6.8)		(4.5)	(4.0)
Amortization of transition obligation		0		0.1	0.1
Amortization of prior service cost		0		0	(2.1)
Amortization of actuarial loss		6.0		5.3	4.3
Sub-total		19.1		16.3	11.9
Regulatory adjustment		(9.6)		(6.8)	(2.4)
Net pension cost		$ 9.5		$ 9.5	$ 9.5

[1] Includes Alagasco.